A device which detects fluctuations in the rotation speed of an automobile engine crankshaft, and diagnoses whether a specific cylinder of the engine has misfired, is disclosed for example in Tokkai Hei 4-113244 published by the Japanese Patent Office in 1992. This apparatus has a magnetic pickup facing the teeth of a ring gear which rotates together with the crankshaft. The magnetic pickup outputs a pulse signal (Pos signal) which varies according to the passage of the gear teeth. In other words, a Pos signal is output at a predetermined small rotation angle of the crankshaft. The apparatus detects changes in the rotation period of the crankshaft by comparing the time which was necessary to count a predetermined number of Pos signals.
In the case of a six cylinder engine, ignition and combustion occur once in each of the cylinders during the time that the crankshaft rotates twice. If the numbers #1 to #6 are assigned to the cylinders in order of firing sequence, combustion takes place at the same crank angle for cylinders #1 and #4, #2 and #5, and #3 and #6. During the first rotation, the cylinders #1 to #3 fire, and during the second rotation, the cylinders #4 to #6 fire. There are therefore three tooth count intervals during a 360 degree crank angle rotation.
The count start point of the number of teeth in these intervals is determined according to a Ref signal. The Ref signal is detected by a crank angle sensor which detects the rotation of a camshaft of the engine. The crank angle sensor is a sensor which outputs a Ref signal, for example at compression top dead center (TDC) of each cylinder, by detecting a specific rotation angle of the camshaft. Consequently the count start point of the number of teeth is set when a fixed number of Pos signals are counted after output of the Ref signal. When it comes to the start point, the aforesaid predetermined number of Pos signals are counted and the time required to count this predetermined number of Pos signals is set as a misfire determining interval TINT. This value is compared with the interval TINT measured for the immediately preceding occasion when a misfire was determined for the same cylinder, and when this difference is large, it is determined that this cylinder has misfired.
However even when the crankshaft rotates at a fixed speed, a difference may appear in the misfire determining interval TINT depending on the interval due to inconsistencies in the manufacture of the ring gear.
In this connection, Tokkai Hei 4-101071 published by the Japanese Patent Office published in 1992 discloses how the precision of the teeth of the ring gear is learnt for each cylinder, and the measured misfire determining interval TINT is corrected by this learnt value. This learnt value is stored for example in a back up RAM with which a control unit is provided.
In this case also, due to an error in detecting the gear teeth by the magnetic pick up or due to infiltration of noise, it may occur that the TINT measuring start point for the same cylinder may be shifted by an amount equivalent to gear tooth. As a result, the TINT measurement interval shifts, so highly precise data are not obtained even if the measured value is corrected by the learnt value stored in the RAM.
In the case of a six cylinder engine, the change in rotation due to a misfire in the high rotation speed area is of the order of 0.1%, this value becoming smaller as the number of cylinders increases. Hence even when there is a shift of one gear tooth in the measured interval, there is a large effect on the diagnosis result.